Interpretive Summary: Aggression is a major problem for swine production as it negatively impacts the pigs’ health and welfare. The neurotransmitter serotonin is essential for aggression control and is synthesized from the amino acid Tryptophan. Dietary approaches to increase tryptophan in the feed in order to raise serotonin availability in the brain and long-term positive social handling have separately been used to reduce stress in pigs. Our objective was to determine whether a high-tryptophan diet fed to grower (3-month-old) and finisher (6-month-old) maternal gilts (female pigs for reproduction purpose) would effect their behavioral activity and aggressiveness and to test whether adding social handling, or not would change responses. We found that feeding the high tryptophan diet did significantly raise the concentration of tryptophan in the blood at both 3 and 6 months of age. The high tryptophan diet also reduced behavioral activity (alertness, walking, drinking and nosing/rooting the pen) and time spent standing, mostly in 3-month-old gilts, and this dietary effect was most marked in non-handled gilts. The high tryptophan diet decreased the number of fights and overall aggressiveness when fed to the 3-month-old gilts, but had no effect when fed to the 6-month-old gilts. Long-term positive social handling improved growth performance, but had little effect on behavior. Provision of high-tryptophan diet reduced behavioral activity and aggressiveness of gilts, especially at the grower stage, and these results are likely mediated by activation of the serotonergic system in the brain. Inclusion of higher levels of tryptophan in commercial diets could improve swine welfare by reducing aggression.

Technical Abstract:
Aggression is a major problem for swine production as it negatively impacts the pigs’ health and welfare. Dietary approaches such as increasing tryptophan (TRP) ingestion to raise cerebral serotonin (5-HT) – a key neurotransmitter for aggression control, and long-term positive social handling have been used to reduce stress in pigs. Thus, our objective was to feed a high-TRP diet to grower (3 month old) and finisher (6 month) maternal gilts that were either socially handled or not and measure their behavioral activity and aggressiveness. Eight pens, each pen housing six unrelated gilts belonging to 12 litters, were split into two blocks equally balanced for litter, social handling (non- vs. handled) and dietary treatment (control vs. high-TRP). Positive social-handling was applied three times per week, from day 45 until 6 months of age. At 3 months, half of the pens were assigned to control diet while the other half to the high-TRP diet fed ad libitum for a total of 6 days (d 1 to 6); the feeding order was reversed at 6 months of age and balanced for social handling. Body weights were taken at day 1 (prior starting the feeding trial), and at d 6 to determine growth rate. Blood was collected at these same days for analyses of TRP and 5-HT concentrations using high pressure liquid chromatography. Behavior was continuously recorded from all gilts and scan-sampling was applied to determine time-budget behaviors and postures in a 12-h period each day (06:00 to 18:00 h) from day 1 to 4. The evaluation of aggression in the home pen focused on agonistic interactions and bites and head-knocks counts per interaction during three 30-min intervals per day (08:00, 12:00, and 16:00 h) from day 1 to 4. Resident-intruder (R-I) test was carried out for a maximum of 300 seconds at day 5 and 6 to measure aggressiveness, predicted by the latency to the first attack and attack outcomes. A 2 × 2 factorial arrangement of dietary treatment and social handling within age was analyzed by repeated measures of mixed models. Feeding the TRP-added diet raised blood TRP concentration of 3 and 6 mon. old gilts by 180.7% and 85.2% respectively (P < 0.05). High-TRP feeding reduced behavioral activity (alertness, walking, drinking and nosing/rooting the pen) and time spent standing, especially in non-handled gilts (P < 0.05). The number of agonistic interactions, but not bites or head-knocks per interaction, decreased in grower gilts fed the TRP diet (P < 0.05). Aggressiveness was also minimized in high-TRP fed gilts but only at 3 mon. of age (P < 0.05), which took longer to attack the intruder gilts and displayed fewer attacks on the first day of testing. Long-term positive social handling improved growth performance, and reduced behavioral activity when associated with high-TRP feeding. Provision of enhanced TRP diet reduced behavioral activity and aggressiveness of gilts, especially at the grower stage, and these results are likely mediated by activation of the cerebral serotonergic system.